Drilling fluids, which are also called drilling muds, are complex mixtures of chemicals used in drilling operations for the production of hydrocarbons and natural gas from subterranean reservoirs. Typically, oil and gas wells are drilled using drilling equipment in the presence of a drilling fluid.
Drilling fluids, generally comprising a liquid or a solid suspension in a dispersing liquid phase, are pumped inside the drilling shaft and exit from the drilling bit through small openings. The drilling fluids return to the surface through the small annulus between the outside of the drilling shaft and the bore hole wall.
Drilling fluids perform a number of functions. Exemplary of these functions are carrying drill cuttings up to the surface and suspending them when the fluid circulation is stopped; cooling and lubricating the drill bit; creating hydrostatic pressure to avoid uncontrolled blow outs and to help supporting the weight of the bore hole walls; and acting as lubricant between the drilling bit and the bore hole walls.
Drilling fluids, moreover, create on the bore hole walls and eventually on the surface of porous geological formations a filter cake having low permeability. The liquid permeating the filter cake and the formation is called “filtrate”.
If the amount of filtrate passing into the formation (filtrate loss) is high, the composition of the drilling fluid itself changes, and this causes loosing control of fluid properties. At the same time, a large fluid loss can cause the deposition of a thick filter cake on the bore hole walls that reduces the diameter of the well bore.
For all these reasons, it is generally desirable that the drilling fluid exhibits a low level of filtrate loss. It is especially desirable that the drilling fluid exhibits a low level of filtrate loss while having specific rheological characteristics of viscosity, plasticity and thixotropy to promote the removal and carrying away of drill cuttings.
Drilling fluids may be classified according to their liquid base.
Oil based fluids comprise solid particles suspended in an oil continuous phase and, possibly, water or brine emulsified with the oil.
Alternatively, water based fluids contain solid particles suspended in a continuous aqueous phase.
Many filtrate reducers for use in oil based drilling fluids have been described, including high molecular weight oil soluble or oil swellable polymers, such as rubber, polybutadiene, polyisoprene, ethylene-propylene copolymers, vinyltoluene-acrylate esters copolymers.
These filtrate reducers are said to minimize the liquid loss through the formation and to inhibit the porosity of the filter cake.
According to a large part of the patent literature, the identity and quantity of monomers in the polymeric filtrate reducers for use in oil based drilling fluids is not too critical.
By way of example:                U.S. Pat. No. 2,805,990 describes drilling fluids having a reduced fluid loss incorporating a latex of polymers of conjugated dienes containing 4 to 6 carbon atoms per molecule, including styrene, 1,3-butadiene, chloroprene, isoprene, methyl pentadiene, 2,3 dimethyl 1-3 butadiene, alpha methyl styrene and halogenated styrene, the polymers being homopolymers or copolymers including terpolymers;        the preferred filtrate reducers of GB 2 131 067 are 100% hydrocarbon polymers and these may be homopolymers or copolymers, even though the hydrocarbon polymers may contain minor proportions of other copolymerisable monomers, such as copolymerisable acids, like acrylic, methacrylic, itaconic or fumaric acids, or amides such as acrylamide, or copolymerisable non-hydrocarbon monomers such as acrylate or methacrylate esters or acrylonitrile;        the oil based drilling fluid of U.S. Pat. No. 7,271,131 comprises a polymer latex capable of providing a deformable latex film on at least one portion of a subterranean formation, the latex comprising polymer particles in an aqueous continuous phase, where the polymer particles are selected from the group consisting of polymethyl methacrylate, polyethylene, carboxylated styrene/butadiene copolymers, polyvinylacetate copolymers, polyvinyl acetate/vinyl chloride/ethylene copolymers, polyvinyl acetate/ethylene copolymers, natural latexes, polyisoprene, polydimethylsiloxane, and mixtures thereof.        
U.S. Pat. No. 3,738,934 describes oil based drilling fluids formed by dispersing particles of oil and water insoluble vinyl toluene-acrylate copolymers which are obtained by reacting (a) vinyl toluene, with (b) an acrylate moiety, selected from the group consisting of methyl, ethyl, 2-ethylhexyl, butyl, dodecyl (meth)acrylates.
In this document, the identity of at least one of the monomers (vinyl toluene) is specified, and the acrylate-to-vinyl toluene ratio (at least 1.65 to 1.00 by weight) is defined; moreover, it is reported that when the product contains a lesser amount of acrylate, the desired swelling does not occur in the water in oil emulsion (oil based fluid).
It has now been found that by co-polymerizing in emulsion 2-ethylhexyl acrylate, styrene and/or vinyl toluene with minor amounts of a polyfunctional unsaturated monomer in specific ratios and conditions, a very effective filtrate reducer for oil based drilling fluids is obtained.
Surprisingly, the presence of a polyfunctional unsaturated monomer and of at least 7% by weight of 2-ethylhexyl acrylate gives a copolymer which is an effective filtrate reducer for use in oil based fluids even when the polymer itself comprises 60% or more of styrene monomer units.